US President Donald Trump slashed the size of two Utah conservation areas Monday, the first such large-scale reversal in more than 100 years.

Trump traveled to Salt Lake City to announce that he was cutting 85 percent of the vast Bears Ears National Monument and around 45 percent of the Grand Staircase-Escalante National Monument.

Trump painted the decision as giving government-held land back to communities and ending Washington overreach.

"The families and communities of Utah know and love this land the best, and you know the best how to take care of your land," he said.

"Your timeless bond with the outdoors should not be replaced with the whims of regulators thousands and thousands of miles away."

The move could open the door to energy exploration and other commercial use.

Trump's opponents say the areas are home to more than 100,000 archaeological sites, including rock art that is at least 5,000 years old and the remains of 21 previously unknown dinosaur species.

The move raises questions about the durability of other conservation areas designated under the 1906 Antiquities Act.

"This law requires that only the smallest necessary area be set aside for special protection as national monuments," Trump said.

"Unfortunately, previous administrations have ignored the standard and used the law to lock up hundreds of millions of acres of land and water under strict government control."

Trump said these two areas were a different order of magnitude.

"I understood how big it is -- I'm a real estate developer. When they start talking about millions of acres, I say, 'say it again? That's a lot,'" Trump remarked.

Environmental lobby group Friends of the Earth accused Trump and his allies of plundering resources.

"Donald Trump is overseeing the largest elimination of protected areas in US history. Dismantling these monuments is Trump's latest gift to the corporate interests who backed his campaign. This action is unprecedented and will end up in court," the group said in a statement.

Meanwhile, 10 environmental protection groups, including the Sierra Club and The Wilderness Society, filed a lawsuit against the Trump administration.

The lawsuit denounced the "unlawful proclamation" which it said "flouted 111 years of conservation history" and would leave natural "treasures exposed to immediate and ongoing harm."

"This is a sad day for indigenous people and for America," Navajo Nation Vice President Jonathan Nez said in a statement, adding the Native American tribe would also file a lawsuit against the Trump administration along with four other tribes.

Source: AFP